A wireless microphone and a microphone-receiver are generally used as a pair of one microphone and one microphone-receiver. Therefore, for example, when a plurality of wireless microphones are simultaneously used at the outdoor concert, the microphone-receivers are prepared as many as a number of the microphones, so that each of used frequencies(channels) is allocated to each of the microphones and the microphone-receivers.
Since a wireless microphone utilizes a radio wave as a communication medium, when a plurality of wireless microphones are simultaneously used in the same area, each of different frequencies should be allocated to each of the microphones and microphone-receivers so as not to cause the mutual modulation-interference(mutual interference).
For example, according to the electric wave-law of Japan, the usable frequency bands for the B type wireless microphones are restricted to thirty (30) channels, into which the frequencies from 806.125 MHz to 809.750 MHz are divided for the particular wireless apparatuses with small electric power in the manner that the frequency-interval between each of adjacent channels is arithmetically equivalent to 0.125 MHz. Each of the different frequencies, which is selected from these limited number of the channels, should be allocated to each of microphone-receivers so as not to cause the mutual modulation-interference.
Since the mutual modulation-interference is caused by the harmonics of radio waves, the combinations of the frequencies which may be used so as not to cause the mutual modulation-interference are grouped by a simulation in advance, and the grouped frequencies are listed on a table. With reference to the table, each of the frequencies is hand-operatedly set in each of microphone-receivers on-site.
When grouping the frequencies, the simulation takes account of interfering radio waves, as well as the frequencies so adjacent as not to be removed by an electric filter of the microphone-receiver and two frequencies inputted into a super-heterodyne microphone-receiver, of which the difference corresponds to the intermediate frequency, and the like.
However, since the frequencies in each of the groups are not always lined in arithmetic order, it is not easy to read the frequencies. This is one of the reasons why wrong frequencies tend to be set in the wireless microphone-receivers, and a great number of the microphone-receivers takes much time to set frequencies.
In the country such as the United States of America, the wireless microphone system is applied by utilizing empty channels of TV. In this case the combinations of the frequencies which may be simultaneously used are incomparably greater in number than in Japan, so that it is considerably a big job to set each of the frequencies in each of the microphone-receivers with reference to the combinations of the table.
Further, even if the channels are set in the microphone-receivers, each time, the existence of the interfering radio waves should be checked by the electric field-intensity-meter such as a RF level meter provided in each microphone-receiver. when especially used on the concert tours, since the state of the other communication systems or the interfering radio waves is different at each of places, it will be happened that the channels should be re-set in the microphone-receivers at each place.